1. Field of the Invention
The present invention relates to a technique for forming, in a case of forming a color image by superimposing a plurality of colors of plane images, a high-precision color image without any positional displacement or aberration of each plane image.
2. Related Background Art
FIG. 10 is a schematic sectional view for explaining structure of a conventional image formation apparatus, and also showing a color laser beam printer which is obtained by applying an electrophotographic technique to a printer.
Hereinafter, the structure and operation of such image formation apparatus will be explained.
An image signal which has been sent from an external host computer 1000 and described in a predetermined description system is inputted into a controller unit 14 of the printer, and converted into a signal capable of being printed. After then, the converted signal is inputted into a laser driver 61 through a CPU (central processing unit) 60. The laser driver 61 modulates light generation intensity of a laser beam and outputs the modulated beam on the basis of the converted signal. An optical path of the laser beam is adjusted by a reflection mirror 62, whereby the beam is irradiated onto a photosensitive drum 12 to form a latent image thereon. In the latent image formed on the photosensitive drum 12, on a portion at which the laser beam has been irradiated, an image selectively appears by voltage applied to development sleeves of development units 64a to 64d and an electric field formed responsive to surface potential on the photosensitive drum 12. Density of the image is substantially determined by field intensity of such electric field and toner amounts of the development sleeves. Reference numeral 63 denotes a charger which uniformly charges the photosensitive drum 12.
The image appeared on the photosensitive drum 12 is once transferred to an intermediate transfer belt (to be referred as ITB hereinafter) 73 by a primary transfer roller 75, to form a reversed image. By repeating such an operation for yellow (to be referred as Y hereinafter), magenta (to be referred as M hereinafter), cyan (to be referred as C hereinafter) and black (to be referred as Bk hereinafter), a reversed full-color image is formed on the ITB 73. In this operation, the ITB 73 is driven by rollers 74 to 76 and 78.
A transfer member put in a cassette 65 is picked up one by one by a paper feed roller 66, and writing timing is adjusted by resist rollers 67. Then, the full-color image which consists of Y, M, C and Bk and has been formed on the ITB 73 is transferred to the transfer member by secondary transfer rollers 77.
Then the transfer member fed to a carrying belt 68 is sent to a fixing roller 69 and a pressing roller 70. By fixing the toner onto the transfer member with the fixing roller 69 and the pressing roller 70, a permanent fixed image is formed on the transfer member, and this transfer member is stacked up on a tray 71. In this operation, the controller unit 14 divides the image signal into process colors (ordinarily Y, M, C and Bk) which are managed by the printer, and sends a drawing signal based on each toner to the laser driver 61. A color reproduction range (or gamut) is determined based on density of each toner and an overlapping manner of the toners. Further, in order to control positional displacement (or aberration) among the colors, various methods have been attempted to prevent a drawback such as teeth blowing of a driving gear or the like by increasing fixing intensity of the photosensitive drum 12 and the ITB 73 and by attaching a fly wheel (not shown) to the photosensitive drum 12.
As a method of controlling the density of each-color toner, there has been provided a method in which an optical sensor 11 constituting of a light emission element such as a light emission diode (LED) or the like and a light reception element such as a photodiode (CdS) or the like is provided to measure or detect the density of the toner transferred onto the photosensitive drum 12, and a measured value is transferred to the controller unit 14 to control development bias, thereby suppressing dispersion in the density due to a change in circumstances or the like. Further, there has been provided a method in which the optical sensor 11 is mounted on the ITB 73 to control the density.
However, in the image formation apparatus having this conventional structure, several problems have occurred. That is, in the conventional apparatus, it has been necessary to apply the fly wheel, to increase accuracy in individual parts, and the like, in order to reduce color displacement (or aberration) by achieving rigidity reinforcement of the apparatus, thereby increasing manufacturing cost, decreasing throughput, and the like. In addition, there has been a further problem that, even in the apparatus of which color displacement level is low in its initial use state, the color displacement increases due to the change in circumstances, endurance and the like a long period of use.
In any case, a main object of the above-described color printer or the like is to print a color image which has been transmitted from an electronic apparatus such as a personal computer or the like. However, in recent years, a function of the electronic apparatus has increased, as a function of the CPU has increased, whereby it becomes possible to cause the electronic apparatus to execute the function of the color printer instead of such the printer. Also, in recent years, communication speed between the apparatuses has increased, whereby it becomes possible to communicate image data at high speed.